Recently, in the fields of auto parts, building material parts, furniture parts, etc., it has been demanded to lighten the weight as much as possible in a state securing rigidity. As one means for this, making the materials hollow is effective. On the other hand, these parts are increasing being bent in view of the needs for arrangement in small spaces, aesthetic design, assembly of a plurality of parts, etc.
There are very many types of bending methods of tube materials. If giving several examples from “Tube Forming”, page 36 to page 64 (Oct. 30, 1992, Corona Publishing Co., Ltd.), there are draw bending (see FIG. 1), ram bending (see FIG. 2), press bending (see FIG. 3), and the like.
Among these, draw bending is the method most generally being used. The advantage is that a wiper die, mandrel, pressure die, etc. constrain the tube material, so there is resistance to wrinkling or buckling at the inner side of bending and bending by a small bending radius is possible. However, put another way, when performing one type of bending, there is the disadvantage that many dies become necessary. Further, bending by a small bending radius is a strong point, but when bending by a large bending radius, a large rotary bending die becomes necessary. Further, it is necessary that the apparatus itself be enlarged. To avoid enlargement of the apparatus, by practice has sometimes been to performing bending by a small bending radius and linear shaping repeatedly to make the overall result close to that of bending by a large bending radius, but this means a plurality of bending operations, so the cycle time becomes longer and the productivity is therefore no good. Further, there are the drawbacks that the bent shape is only a circular arc and further in principle bending by only one type of bending radius is possible.
On the other hand, ram bending includes the system as shown in FIG. 2 of using a bending die and support rollers and also the case, as shown in FIG. 4 (“Journal of the Japan Society for Technology of Plasticity”, Vol. 44, No. 508 (2003), page 530), where the support points do not rotate. There are the advantages that ram bending, compared with the aforementioned draw bending, requires fewer dies and, further, because bending is possible with just the movement of a punch (the bending die in FIG. 2), the productivity is high. However, there is less constraint by surrounding dies, wrinkling and buckling easily occur at the inner side of bending. In particular, when the distance between the support points is large, buckling such as crumpling easily occurs at the location pressed by the punch.
Press bending is a method as shown in FIG. 3 which bends a tube material while a pressure die rotates around a bending die. It is relatively similar to the aforementioned draw bending, but they differs in whether the bending die rotates or whether the pressure die rotates. For the pressure die, other than when using a die such as in FIG. 3, there is also the example of utilizing a roll such as in FIG. 5 (Japanese Patent Publication (A) No. 3-32427) (note that in FIG. 5, (a) to (d) show, respectively, FIG. 1 to FIG. 4 of Japanese Patent Publication (A) No. 3-32427, in which 1 is a fixed die, 2 is a guide surface, 3 is a groove, 4 is a support shaft, 4a is a pinion rack, 5 is a press fluid pressure cylinder, 6 is a bearing frame, 7 is a pressure die, 7a is a spindle, 8 is a groove, 9 is a rotary fluid pressure cylinder, 10 is a hole type die, P is a material tube, and Pa is a front end part). However, the drawbacks that the bending shape is limited to a circular arc and that bending by a large bending radius is difficult in terms of equipment are similar to the case of draw bending.